Fig. 2: Following dark anoxia, light exposure longer than 10 s gradually slows down electron output from PSII.

a Mixotrophic C. reinhardtii wild-type strain CC124 cells were tested for H₂ evolution in a MIMS. The cells were incubated for an hour under dark anaerobiosis, after which they were illuminated (370 µE m⁻² s⁻¹) for a duration of either 5, 10, 20, 30, 45, 60, 120, or 180 s (Exposure I, see bolted time scale). Presented here is the trace, which was measured by exposing the cells for a duration of 45 s in exposure I. Following the initial light exposure, the cells were kept in a darkness for 3 min (gray background) and illuminated again for 2 min, three times (Exposures II, III, and IV, white background). To compare the effects of the duration of exposure I on photosynthetic regulation, results which were measured during exposure II were plotted against the accumulated concentration of H₂ (b) and photosynthetic efficiency, which was assessed by Chl a fluorescence measurements (c). Electrochromic shifts were determined by measuring changes in cells’ absorbance (520–546 nm). Three seconds following exposure I (see dashed arrow in panel a), the cells were exposed to a 5 ns laser flash, and charge separation was measured (d). The color index in all panels matches the duration of exposure I (ranging seconds): 5—purple, 10—blue, 20—cyan, 30—green, 45—light green, 60—yellow, 120—red, and 180—dark red. Each experiment was repeated using at least three biological replicates. Error bars indicate standard error (n ≥ 3).